1. Field of the Invention
The present invention relates to a combustion apparatus for use in a heating system, hot water supply system, air-conditioning system, portable heater and other equipment in which such gaseous fuel as natural gas and propane gas or liquid fuel such as kerosene and light oil are burnt for providing a heat source.
2. Related Art of the Invention
Catalytic combustion is a method of burning a fuel-air mixture by using a catalyzer with a platinum alloy carried by such ceramic carrier as honeycomb and fiber.
A catalyst used for combustion has a selective adsorption to oxygen and hydrocarbon, and allows them to react with each other on a surface of the catalyst. In such operation, because the catalyst is at a temperature lower than that obtained by flame combustion of an identical gas, almost no NOx is produced. It is a problem, however, that the temperature of a catalyzer is increased to 1,200.degree. C. or a higher temperature, if a combustion apparatus using a catalyst is operated at a combustion load (intensity of combustion in relation to a volume of combustion chamber) identical to that of a flame combustion apparatus, and a life of the catalyst in terms of heat resistance is significantly reduced. It is, therefore, required to use the catalyst at a temperature lower than a critical temperature of heat resistance thereof by using means for reducing the combustion load and increasing the size of a combustion chamber or employing means for increasing the excess air ratio in a fuel-air mixture and reducing the combustion temperature.
Flame combustion is achieved at an excess air ratio of 1 to 2. On the other hand, catalytic combustion is achieved at an excess air ratio of 1 to 5, and it, therefore, allows use of a leaner fuel-air mixture. However, it is a problem that a thermal efficiency is considerably lowered, when a leaner fuel-air mixture is employed. It means that a difference in temperature between a heat exchanger and combustion exhaust is reduced, because a combustion temperature is lower at a lower concentration of fuel, and a rate of heat transfer is reduced. Thus, in order to obtain a higher heat efficiency, a heat exchanger of a larger size is required, and it has been difficult to provide a compact catalytic combustion apparatus having a high combustion capacity.
Additionally, in catalytic combustion, although it is required to preliminarily mix the air with a fuel to cause a reaction, it is a problem, when a liquid fuel is employed, that a higher heat is required for vaporizing the fuel. In a conventional liquid fuel combustion apparatus of vaporization type using the flame combustion method, although a vaporizing unit is heated by an electric heater only at an initial stage of the combustion, consumption of an electric power is low, because it is heated by applying a flame to a part of the vaporizing unit during stationary burning. In the case of a conventional combustion apparatus using the catalytic combustion method, however, it is a problem that an electric power supply is required for vaporization heat even in a stationary state, as no flame is formed, resulting in an additional power consumption.